E. cuniculi is a Microsporida, an obligate intracellular parasite, that occurs frequently in numerous mammals and is implicated in various infections in humans, principally in immunodepressed subjects. Two other species of the genus Encephalitozoon (E. intestinalis and E. hellem) are also implicated in various opportunistic infections. Generally speaking, the microsporidians are responsible in AIDS patients for gastrointestinal diseases, as well as ocular, muscular and hepatic disorders, rhinosinusitis and systemic infections [1]. Serological tests have also demonstrated the noteworthy presence of microsporidians in immunocompetent patients at the level of 8% of the population [2]. Four genera of microsporidians are responsible for human diseases: Enterocytozoon, Encephalitozoon, Vittaforma and Trachipleistophora. The emergence of these parasites in human pathology has created an increasing interest on the part of researchers in the systematic, epidemiological, clinical, diagnostic and therapeutic fields.
At present, diagnosis is based on PCR tests from oligonucleotides determined according to the ribosomal DNA sequences, the sole sequences known in most of the microsporidians. With regard to therapy, it is limited to certain compounds such as albendazole and fumagillin.
These unicellular eukaryotes exhibit a unique invasion mechanism. The spore, which is the infectious stage, contains an extrusion apparatus constituted by a polar tube inserted at its anterior end in an anchorage disk. Under the effect of certain stimuli, which can be linked in vitro to a variation in the pH, osmolarity, or the presence of cations or anions, the polar tube is extruded by the microsporidian spore and traverses the plasma membrane of a cellular host. The sporoplasm, expelled via this tube, is thereby inoculated into the receptor cell. These invasive apparatus, specific to the microsporidians and unique in the world of living organisms is, therefore, of great interest from not only the fundamental point of view but also from the applied point of view for diagnostics and therapeutics.
To date, no complete sequence of proteins constituting this polar tube has been obtained. According to Weidner [3], the polar tube is constituted by a single 23-kDa protein in Ameson michaelis. More recently, in a microsporidian parasite of fish, Glugea americanus, a differential extraction of the proteins in the presence of a reducing agent (DTT) made it possible to demonstrate that a 43-kDa protein is constitutive of the polar tube but only a part of the N-terminal sequence of 16 amino acids was determined [4]. Production of polyclonal and monoclonal antibodies against the polar tube of various species [5, 6, 7] has also been performed, thereby demonstrating a possible protein heterogeneity of this structure.